When imaging is performed with a film or imaging device of an imaging apparatus irradiated with a small amount of light per unit time, the imaging device or film needs to be irradiated with more amount of light by setting a long exposure time such as several tens of seconds, several minutes or longer. Examples of performing such imaging may include performing imaging in the dark, performing imaging on a dark object such as a star, and performing precise imaging with narrow aperture and deep depth of field.
In imaging devices, such as CCD (charge coupled device) or CMOS (complementary metal-oxide semiconductor) imager, that convert irradiated light to electric signal by photoelectric conversion to output, imaging signals output with a predetermined period (such as frame rate) are added for each pixel to increase dynamic range. JP-A-5-236422 describes a configuration for accumulating image signals obtained from an imaging device for each frame to increase dynamic range.
Under imaging conditions such as long exposure, increase in the number of white spots in the dark and increase in the signal level due to a fixed pattern of the imaging device may occur, which may cause image data obtained through imaging to be very noisy and visually undesirable. This is because of the following reasons. A white spot caused by leak current occurring in a pixel section of the imaging device (hereinafter referred to as white spot) is due to a fixed pattern specific to the imaging device. The signal level of the white spot increases in proportion to exposure time. Accordingly, the signal level of the white spot, which would be around black signal level by short exposure, may become visible-signal level by long exposure, then causing exponential increase in the number of visible white spots in the dark.
When the signal level of a white spot caused by leak current is increased, by addition, to a level that exceeds memory word length, the information of the pixel with the white spot is clipped at the upper limit of dynamic range, which becomes a defect. When the number of white spots in a screen is sufficiently small, this problem can be solved by performing interpolation using adjacent pixel signals.
However, as described above, long exposure may cause many white spots, in which one white spot is likely adjacent to another white spot and then the interpolation using adjacent pixels may not improve image quality. This requires canceling white spots while increasing dynamic range by adding imaging signals.
One conventional method for solving this problem cancels white spots using image signal obtained by performing imaging with an imaging device shielded from light. For example, first, imaging is performed with the imaging device irradiated with light from an object, then imaging is performed for the same exposure time with the imaging device shielded from light, and then the image signal obtained with the imaging device shielded from light (hereinafter referred to as “light-shielded image signal”) is subtracted from the image signal obtained with the imaging device irradiated with light (hereinafter referred to as “non-light-shielded image signal”) for each pixel. This method can cancel white spots, because, in light-shielded image signal, the white-spot signal level of a white-spot pixel increases according to exposure time, while the signal level of non-white-spot pixels are ideally black signal level.
JP-A-2003-219282 and JP-A-8-51571 describe techniques of removing a fixed pattern noise specific to a given imaging device by subtracting light-shielded image signal from non-light-shielded image signal for each pixel, as described above. In addition, according to JP-A-8-51571, white spots are more appropriately removed by reducing the level of light-shielded image signal to 1/N and repeating the subtraction of the 1/N reduced light-shielded image signal from non-light-shielded image signal N times.